1. Field of the Invention
This invention relates to an external-measured real image type finder optical system provided with an objective lens for its own use, and more particularly to a viewfinder optical system which is arranged to permit correction of the parallax of the finder field.
2. Description of the Related Art
A real image type finder optical system arranged to permit an inverted finder image to be sighted as a non-reverse erect image by means of a Porro prism, or the like, has been known. An example of the viewfinder of this kind has been disclosed in Japanese Utility Model Publication No. SHO 47-19028, etc.
Compared with a reverse Galilean finder, or the like, arranged to give a virtual image of an object to be photographed, a real image type finder optical system generally permits sighting a clearer, brighter field image and a clearer field frame. Besides, the latter permits the optical system to be more compactly arranged. Hence, the real image type finder optical system is often used for a compact camera.
FIG. 1 of the accompanying drawings shows in a side view a typical real image type finder optical system. In FIG. 1, reference numerals 100, 101, 102 and 103 respectively denote an objective lens, a Porro prism, an eyepiece lens and a field frame positioned at an image plane.
During recent years, an electronic camera of the kind having a solid-state image sensor such as a CCD has become practicable. The image sensing plane of the solid-state image sensor of the camera of this kind is of the so-called 1/2 inch size measuring 6.4 mm by 4.8 mm. Compared with the ordinary 35 mm film size, the size of the image sensing plane is about 1/5 of the former in length. Even with an allowable degree of parallax assumed to be about 5%, the camera of this kind, therefore, must be arranged to have a deviating degree of field less than 0.24 mm in the shorter side direction of the image sensing plane. The parallax correction would be very difficult if the camera of this kind is arranged to have the same degree of precision for parts and assembly as the ordinary 35 mm size camera. Particularly, for the above stated real image type finder optical system which permits clear sighting of the field and the field frame, this problem is serious. This problem, therefore, necessitates a severe allowable degree of parallax.